"Brian Sponcil" writes:

I have a '75 151 with similar problems. Basically my Ammeter reads normally
(10 amps) while on the ground and through run-up. However once I'm in the
air for a little while it drops to zero and no amount of cycling the alt
switch fixes it. I replaced the voltage regulator but the only effect I saw
from that was a "calming" of the Ammeter needle. I suppose that was worth
$100 ;-) Anyway, I suspect that the Chrysler alternator, a rebuilt unit
installed at the last annual, is the culprit. I'm having the A&P look at it
monday so I'll get back to you with the verdict.

Depending on the meter, this sounds quite normal.

As the other poster said, there are 2 different ammeters out there.
A zero-center one reads current into/out of the battery. A single-ended
one reads output from the alternator. Not the same thing at all.

The zero center flavor will start out a flight by showing a discharge,
then after you start the engine, it should start to charge. After
you make up for the battery's amp-hours used in starting, it should
drop to zero -- the battery is charged and needs no more.

The alternator output meter is a different story. It should start
out showing some output at idle/taxi [recharging], then lots at
climb/cruise, [still recharging but more RPM=more amps possible]
then NEAR zero once fully charged. How far from zero depends on what
you else are running. (If you have a PL-12 with 3 navcoms,
2 transponders, one ADF, XM radio, a LORAN and twin GPS, it will be
a bit more than Ron's Fly Baby.)

Note the "zero" is not exact on either gauge.

There's an easy test while in flight. Turn on some load - the landing
lights will do nicely. The zero-center battery gauge should stay
the same, provided you've not exceeded the alternator capacity
{unlikely}. The alternator-output gauge should show 10+ amps more
output & drop again when you turn off the lights.

The OP mentioned the alternator-output pegged when he brought it
back on line. That's not unexpected. The alternator was attempting
to recharge a depleted battery. Safe? I've never seen an automotive-style
alternator damaged by overload. I suppose already-weak diodes might
fail; but in general, the output is limited by the available magnetic
field. That's limited by the field current, set by the voltage
regulator -- by changing the field current, it changes the field
strength, and that changes the output voltage. The regulator can't
do more than put a full 13v on the field, and that's within spec.

And the bigger lesson is to learn how your a/c normally acts, and
notice when it changes.



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